Reference boolr not to be

taken from the library.

^eckniccil ^^ote 169

PROFILES OF ELECTRON DENSITY

OVER THE MAGNETIC EQUATOR

OBTAINED USING THE

INCOHERENT SCATTER TECHNIQUE

K. L. BOWLES et al.

' U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS

^ecUnicai 169

ISSUED MARCH 16, 1963

PROFILES OF ELECTRON DENSITY

OVER THE MAGNETIC EQUATOR

OBTAINED USING THE

INCOHERENT SCATTER TECHNIQUE

K. L. Bowles

Central Radio Propagation Laboratory

National Bureau of Standards

Boulder, Colorado

and

Staff of Jicamarca Radio Observatory

Instituto Geofisico del Peru

Lima, Peru

NBS Technical Notes are designed to supplement the Bureau’s regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 25 cents

Profiles of Electron Density Over the Magnetic Equator

Obtained Using the Incoherent Scatter Technique

K. L. Bowles

and

Staff of Jicamarca Radar Observatory, Lima, Peru

A joint venture of the National Bureau of Standards, Boulder, Colorado,

and the Institute Geofisico del Peru, Lima, Peru

The technique of electron density measurement by incoherent

scatter, using a high powered radar, has now been thoroughly explored

in the literature. A representative bibliography on the subject, as well as a summary of experimental aspects, current early in 19^2, has

been given by Bowles, Ochs, and Green [1962] . The object of this note

is to present a number of profiles obtained using incoherent scatter

near Lima, Peru.

It appears that the theory of the "incoherent scatter" phenomenon,

as applied to the ionosphere and exosphere, is now well established.

The scattering cross section per free electron, a, is related to the

classical Thomson cross section a,, by the approximate formula

x>n

o = ath

1

1 + T /ti-J

as long as \ < < kTt\

D

( 1 )

( 2 )

where T^ and T^ are the electron and ion kinetic temperatures, respectively, X is the radio wavelength and is the Debye shielding distance.

For all of the experiments to be disucssed in this note, the inequality

(2) is correct.

1 -
-2 -

At first there appeared to be some question as to the validity of

(l), raised by some of the experimental results. The paper of Bowles

et al. [1962] showed that the scattering cross section measured experimentally is within about 20 percent experimental error of agreeing

with the theory, at least under conditions of thermal equilibrium.

Recently Renau [1962] has questioned the earlier theories, all of which

led to (1), and suggests that o as increases. Unfortunately Renau 's theory leads to the inconsistent result that the electron

and ion fluctuations behave quite differently as changes, a result which is known to be impossible from the theorem of charge

neutrality. Quite recently Evans, in work which will be published

shortly [j. V. Evans, private communication, I962], has obtained

accurate spectra of incoherent scattered echoes observed in Massachusetts

These spectra agree precisely with the earlier theoretical spectra, which

correspond with (l), and apparently do not agree with Renau 's spectra.

Thus we assume, for the present purpose, that (l) is correct.

Now the ratio T /t. can be measured from the incoherent scatter

e' 1

echoes by observing the spectrum of the scattered energy. Unofrtunately

| we were not prepared to make such measurements when the profiles given

herein were obtained. We estimated [Bowles et al., I962] that at Lima,

and at the F2 layer maximum, T^/T^ approximates unity during the

majority of the daylight hours, and approximates 2 just at sunrise in

the F region. We had very little data for the nighttime hours. On the

other hand, [Evans, private communication, I962] finds that in

Massachusetts approximates unity during the hours of darkness and

rises to about 1.6 at midday at heights from 3OO to 6OO km. Other

reports also seem to indicate that thermal equilibrium is by no means

necessary and that values of T^/t^ approximating 2 or more are quite

}

I

common.
-3

-

I The profiles given in this note are therefore subject to whatever

lerrors might be introduced by variations in T /t. with height at any

! / e 1

.given time. Obviously if does not vary with height, a is the

jsame at all heights and the profiles are plotted correctly.

i Only one point on the profiles is directly related to the correct

lelectron density. This is the point of maximum electron density, the

j value of which is obtained from the ordinary -trace penetration frequency

jof a conventional swept -frequency ionogram. All other points are

I relative measurements normalized to the maximum value. The error flags

shown on the profiles give only the relative accuracy for scaling the

incoherent scatter echo power which is proportional to the electron

density at each height. No errors due to variations of

I shown

It may be seen that most of the profiles were obtained in three

distinct sections. Error flags appearing in the vicinity of TOO km

height pertain to the section of the profile which also includes the

P layer maximum. The incoherent scatter technique uses direct measurements of echo power. The sensitivity of the radar decreases with

height h as l/h . Therefore the measurement involves observations over

a dynamic range of more than 50 decibels of received power. This

requires operating the radar at three different sensitivities in

j succession in order to obtain the entire profile. At the higher

1 sensitivities measurements cannot be made at the F layer maximum due

ito saturation of the radar receiver. Therefore the complete profile

j is a composite obtained by fitting sections of profile obtained at

i three different sensitivities. The accuracy of the higher sections

obviously depends upon the accuracy of curve fitting. Readers may

I judge for themselves whether the fitting has been done properly,

j since data points pertaining to each section of the profile have been

[plotted using distinct symbols. In some of the profiles, all of the

1

I available data points have been plotted individually. In others only a

j few have been plotted as a means of identifying the three sections of

profile
-4 -

At times coherent irregularities are present at certain heights '

giving rise to radar echoes stronger than the background of incoherent li scatter. The most obvious of these are associated with the appearance ;ls of spread-F echoes on the conventional swept frequency ionograms. When ,.! the spread-F is intense it has proven impossible to obtain reliable t

j electron density profiles. At times of weak spread F we have excluded i.i] the range of heights corresponding to the spread-F echoes from the pi

electron density analysis. Now it may logically be asked whether our h

observations are not always contaminated to some extent with scattered I power due to coherent irregularities. Having been unable as yet to make i adequate spectrum obseiwations we have not yet been able to exclude the

coherent echoes from the electron density observations with absolute

certainty. However several factors lead us to believe that the coherenijji

’•'!l irregularities are normally of trivial Importance. First, the coherent

h

j irregularities in the E region and above are always obseirved to be

quite aspect sensitive and aligned with the lines of force of the

earth's magnetic field. Therefore the echoes due to these irregularitie

must maximize in an angular region within about 1 degree of perpendicular to the lines of force, according to our spread-F obseinrations .

*'!

i Except when spread F is also observed with the ionosonde, there is no

''

f evidence of significant echo power from coherent irregularities even i

'll when the radar antenna is directed perpendicular to the magnetic ^ field line in the F region. All of the profiles of electron density ^ reported herein were obtained with the antenna directed 3 degrees away ' from perpendicularity. Under this arrangement the coherent echo power

should be reduced perhaps three to four orders of magnitude relative to

the perpendicular direction. Therefore we feel justified in ignoring 1 the coherent irregularities for analysis of most of the profiles pre- |

sented herein. Exceptions are noted directly on the graphs.

1
- 5

-

I At heights helow about 200 km there is some evidence that the

I indicated electron densities are up to a factor of 2 too high. This is

isuggested by comparisons made using the so-called true height method of

ianalysis of conventional ionograms. Below 200 km the mean free path of

jthe gas becomes short enough that coherent irregularities due to turbulence and other hydrodynamic sources may become of some concern. Also

the role of collisions in the incoherent scatter process is not yet well

lunderstood. We are making efforts to understand the scatter from this

height region better. For the present it must be assumed that the protfiles given in this note are reliable only above 200 km, and that the

electron densities indicated below that height are overestimated.

I Most of the details of our technique in making incoherent scatter

profile measurements have been given in the paper by Bowles et al. [1962]

iln the profiles given herein, the transmitter peak power P, and antenna

cross section A were as follows

January I962 P. = 10^ watts A = 2.1 x lO^m^

February I962 P^ = 2 x 10 watts A = 4.2 x 10^

April 1962 et seq. P^ = 4.5 x 10^ A = 8.4 x lO^m^.

In February and following, the top section of the profile was extended

through the use of longer pulses (up to 3 milliseconds) and improved

jin integration of the receiver output.

i Our experimental program includes the measurement of the incoherent

scatter spectrum in order to determine T^/t^. Equipment to permit this

jis now essentially complete. Every effort; will be made to produce

[profiles in greater quantities and for longer periods of continuous

operation.

I

I

!
-6 -

Bibliography

Bowles, K. L., G. R. Ochs, and J. L. Green, On the absolute intensity

of incoherent scatter echoes from the ionosphere, J. Res. NBS 66p

(Radio Prop.), 395 -^0? ( 19^2 ).

Renau, J., The cross section for scattering of electromagnetic waves from an ionized gas in themial nonequilibriTim, J. Geophys. Res. 6j,

3624-3526 (Aug. 1962).
I -7

-

-9

-

poo _

000 _ poo _

!jooo _

i

I|ooo -

l500 -

000

500

000

900

800

700

600

500

400

300

200

1 1—I I r rr r

i

1 1—i i ii i

1

1

1 1—i—i i i n |1 1—i—

150

100 j I

I I I I j I I I I I I I I

I I I I I I I I I L 10 = 10 lO" 10

ELECTRON DENSITY (N/cmM

III

I I 1 1 1

10^
HEIGHT

(Km )

-10
HEIGHT

(Km )

- 11-

_6000

-5000

_4000

_3500

_3000

- 2500

_2000

- 1500

.1000

900

800

700

. 600

.500

-400

-300

FACTOR t<F<-|-

DUE TO THE NEED TO ASSUME

T*

\

\

)

Ti •>1

- ZOO

. 150

10^ 10 ^

, 10

' ELECTRON DENSITY (N PER cm®)
HEIGHT

(Km )
HEIGHT

(Km

- 13-

- 15-
7000

6000

5000

4000

3000

2 500

2000

1500

1000

90 0

800

700

600

500

400

300

200

1 50

iOO

-16-

I 1 1 1

1

1

1

I—I—I -I I II

-J 1 L I I M I I I- I I I I I r I 10

^

-I 1

I I I I 1

J 1 I

I I I I I 10 10

=* ELECTRON DENSITY (N/cm®)

10
HEIGHT

(Km )

- 17 -

ELECTRON DENSITY (N/cmM
18.

I
19

ELECTRON DENSITY (N/cm’)
-20

7000

1 1—I—1—1 1 1

1 1

1 1—1—r 1 » t

«

]

» 1—*—1 I I \

1—r-r

6000

5000 = ^

""

' LIMA, PERU

4000

\ 17 FEB. 1962

H \ ^ 1 1340 EST

3000

\ M 1330 EST

\ 1320 EST _ 1 )«—V

c,—

\ 1260 EST

2500 JUh

200 0

r \

1500 - 1 0^

1000

"Z 900 - \

J 800 O 1 700

S 600

^ 50 0

X

400

300

200

o

_

150

a

— o

a

100 \ 1 1

1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 III!

lo’ lo'* 10® 10®

ELECTRON DENSITY (N/cm’)
-21-

I I I I I I 1 1

I I I I I II I i 1 1—I I I 1 ii.i 10* 10* io"* 10* 10*

ELECTRON DENSITY (N/cm®)
HEIGHT

(Km )

-22 -
-25-

1500 -

HOOO

900

800

700

600

500

400 I300

200

1 50

-| 1—r-rm-rp I I I M I I TTTTn 1 1—I

n" i ^ r

LIMA, PERU

12 JAN. 1962

2300 EST

2320 EST

100

10

" 10 10 10

ELECTRON DENSITY (N/cm’)

10

I
HEIGHT

(Km )

-2U- I

1

I rr'

i Ti I\

!—I—I I M 1

1

1 1—I ‘1

" rnTi 1 1—r"TTT

1500 _

1000

900

800

70 0

6 00

500

400

300

200

150

100

10

LIMA, PERU

12 JAN. 19 62

2240 EST — 2250 EST

j—I—I—1 1 1 1 1 1 1

i I

I I I i 1

I I I I i 1 1

j I I I ill 10 10 10

ELECTRON DENSITY (N/cm*)
25 jjI
HEIGHT

(Km)

-26-

150 0

1000

900

80 0

70 0

600

500

40 0

300

200

150

LIMA, PERU

12 JAN. 1962

_K>_ 1810 EST (1 -wi— 1815 EST

J

10 0

10

' 10 10 iO

ELECTRON DENSITY (N/cm®)

1
27 -

ii

;

r j

I

I

i

;!

2 3 4 5 6 10 10 10 10 10
HEIGHT

(Km)

-28
HEIGHT

(Km)

-29-

1500 _

1000

900

800

700

600

500

400

300

200

1 50

1 I

I r r ri 1 1

1 1 i 1 1 1 1 1 1

1 1 r 11 rn

100 I I I I I 1 1 1 1 I I I I I I 1 1 1 I I I I I I 1 1

j I

10 10 10 10

ELECTRON DENSITY (N/cm^)

iO
HEIGHT

(Km)

30 -

1500

1000

900

800

700

600

500

400

300

200

15 0 _

-I—I T I

I n"|

1—I—I I I I I

1

1

1—\—I 'I I m| 1—I—r-TT

100 -I—I—1 I I I 1 1 1

1

LIMA, PERU

9 JAN. 1962

_o_ 2320 EST

2315 EST

J 1 L. i 1 I I I I

I I I I I

10 10

^* 10" 10

ELECTRON DENSITY ( N/cm*

}

i

* U.S. GOVERNMENT PRINTING OFFICE ; 1963 O—67637'r|
U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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